When Laszlo Tisza first came to MIT in 1941, he had already made significant contributions to physics. In the years since, he has consolidated his position as one of the most important theoreticians of his time. Tisza's major areas of activity, closely reflected in these twenty-three essays, have included studies of quantum liquids (in particular, the remarkable properties of liquid helium and the nature of superfluidity and superconductivity), irreversible thermodynamics and the statistical thermodynamics of equilibrium, phase transitions and critical phenomena, and the application of group theory to molecular spectroscopy.Tisza has also given long and close attention to the philosophy and history of his science, to a degree rarely attained by an active research physicist. His special contribution has been his insights into the logical and conceptual structure of physics. This aspect of Tisza's work is less well known than his technical contributions, and the book has been structured to right the balance by revealing Tisza the natural philosopher who collaborates with Tisza the physicist.Written by Tisza's colleagues and former students, the essays are grouped under five headings: Foundations of Probability and Thermodynamics; Condensed Matter Physics; Quantum Mechanics and Relativity; Biological Systems; and History and Philosophy of Science.Abner Shimony, Professor of Philosophy and Physics at Boston University, has contributed a closing evaluation of Tisza's philosophy of science, and Herman Feshbach, Head of the Department of Physics at MIT, has contributed an opening recollection of Tisza's scientific style.